In recent years, regarding writing a servo pattern by STW (Servo Track Writer), various writing methods are disclosed for the purpose of improving the quality of written servo pattern and shortening the writing time.
(1) Single Direction Writing Method Using Single Plate STW or Stack Servo Track Writer
Conventionally, when a servo pattern is written by the STW, a single direction writing method using a single plate STW or stack servo track writer is used in which a servo pattern is written in a single direction from inner circumference to outer circumference of a medium or from the outer circumference to the inner circumference.
However, in a medium on which a servo pattern is written by using this method, as illustrated in FIGS. 10A and 10B, side fringes due to the written servo pattern are mixed in the outer circumference portion and the inner circumference portion compared with the middle circumference portion. Therefore, a magnetic disk apparatus using such a medium reads a read signal with the side fringe superimposed as noise when demodulating the servo pattern. As a result, the read signal is injected into a servo loop as a RRO (Repeatable Run Out) or NRRO (Non-Repeatable Run Out), so that positioning accuracy deteriorates.
(2) Both Direction Writing Method Using Single Plate STW
As a method for compensating for the deterioration of positioning accuracy in the single direction writing method using the single plate STW, a both direction writing method using the single plate STW is disclosed in which the side fringe is remaining as few as possible as illustrated in FIG. 11 (refer to Japanese Patent Application Publication (KOKAI) No. 2006-260715 and U.S. Patent Application Publication Serial No. 2006/0209452A1). The both direction writing method using the single plate STW is a method in which, for example, the servo pattern is written from the outer circumference to the inner circumference in the outer circumference portion of the medium, and the servo pattern is written from the inner circumference to the outer circumference in the inner circumference portion of the medium. In a magnetic disk apparatus on which the servo pattern is written by using this method, it is possible to selectively use only a portion on which the side fringe remaining in the pattern written on the medium has less effect.
However, the STW using this method is affected by so-called backlash of a head actuator system, since the head actuator while writing the servo pattern does not operate in a single direction, but operates to and fro. The backlash means a mechanical allowance (or looseness) between a motor output and a load, and especially means a mechanical allowance between a voice coil motor and a head. Therefore, the backlash is not a problem for the STW in a mechanism moving in a single direction. However, the STW in a mechanism moving to and fro cannot match the starting point and the returning point accurately due to the above mentioned allowance.
In other words, in the STW in which the head is not positioned directly to the medium, for example, the servo pattern written from the outer circumference and the servo pattern written from the inner circumference cannot be joined to each other correctly at a scheduled radius position in a middle circumference portion of the medium due to the effect of the backlash when the servo pattern is written with the to and fro movement. Specifically, an area in which the servo pattern is not written is generated (refer to FIG. 12), or contrary to this, a pattern written later is overwritten on a pattern which has been written from the opposite direction (refer to FIG. 13), or a pattern at the junction which cannot be correctly demodulated as a servo pattern is written (refer to FIG. 14). Therefore, in a magnetic disk apparatus using such a medium, the head cannot be correctly positioned at the portions illustrated in FIGS. 12 to 14 as described above.
(3) Servo Pattern Writing Method Using Self-STW
As a method for compensating for the disadvantage in the both direction writing method using the single plate STW, a servo pattern writing method using the self-STW is disclosed (refer to Japanese Patent Application Publication (KOKAI) No. 2005-346766, U.S. Patent Application Publication Serial No. 2005/0264917A1, and U.S. Pat. No. 5,668,679). The STW using this method writes a seed track to an innermost circumference to be a reference, and then, performs servo pattern writing toward an outermost circumference direction while controlling the position of the head using the written seed track pattern to form a servo pattern on the entire surface. In other words, the STW increases a gradient of magnetic transition of the servo data, and re-records (re-writes) the servo pattern, whose recording quality deteriorates, from the outermost circumference to the middle circumference.
(4) Operation Method on Junction
As another method for compensating for the disadvantage in the both direction writing method using single plate STW, a method is disclosed in which the servo pattern is not formed on the junction and the head can pass over the junction having no servo pattern by a seek operation (refer to Japanese Patent Application Publication (KOKAI) No. 2004-326966 and U.S. Patent Application Publication Serial No. 2004/0212917A1). In other words, the STW does not write the servo pattern on an area near the portion where the both direction writings join to each other when writing the servo pattern from both directions. The magnetic disk apparatus controls the head to pass over the portion where the both direction writings join to each other without using the junction as a user area by the seek operation.
However, in the above described conventional technique of (3) servo pattern writing method using self-STW, the head positioning quality deteriorates because track misregistration of the re-recorded servo pattern is not improved.
Also, in the above described conventional technique of (4) Operation method on junction, a performance of sequential access deteriorates because the head has to be operated so as not to seek a part of the medium. In addition, in (4) Operation method on junction, a data storage capacity decreases because a part of the medium is not used as a data capacity, and a capacity and a development work of firmware increase because special firmware is required so that the head does not seek a part of the medium.
Specifically, in (3) servo pattern writing method using self-STW, when re-recording (re-writing) the servo pattern from the outermost circumference to the middle circumference, the servo pattern is re-recorded while a head is positioned with respect to the servo pattern whose recording quality deteriorates. Therefore, even though the signal quality itself of the re-recorded servo pattern is improved, the track misregistration of the re-recorded servo pattern due to deterioration of head positioning quality when re-recording the servo pattern is not improved.
In the method of (3), since the servo pattern for a part where the recording quality deteriorates is re-recorded after the servo pattern is once formed on the entire surface, a total servo pattern writing time is increased by a time necessary for re-recording of the part compared with a case that the servo pattern is simply formed on the entire surface. For example, if it is simply assumed that the time required for writing on the entire area=1, and a half the entire area is re-recorded, in order to completely record the servo pattern with the (3) method, the servo pattern writing time is “time required for simply writing on the entire area=1”+“time required for re-recording=0.5”, so that about 1.5 times the servo pattern writing time is consumed. Therefore, the processing time increases, so that, also considering the increase of servo pattern writing time due to high density recording in the future, it cannot be expected that manufacturing efficiency will be improved.